In the context of musical melody and stepwise motion, the most common pattern from the options provided would logically suggest a series of movements that are primarily either whole or half steps.
Considering the options:
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a whole step, two half steps, a whole step: This pattern would include two half steps and two whole steps, leading to a varied melody while keeping it mostly stepwise.
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a half step, three whole steps: This option is somewhat less stepwise, as starting with a half step could create a larger leap compared to the rest of the pattern.
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four half steps: This option consists entirely of half steps, leading to a very tight, chromatic motion, but does not incorporate any larger stepwise motions (whole steps).
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two whole steps, two half steps: This combines a couple of whole step movements with half step movements, which can create a more balanced melody.
Given the above evaluations, "a whole step, two half steps, a whole step" reflects a combination that maintains stepwise motion while creating a melodic contour.
Thus, the answer is: a whole step, two half steps, a whole step.